Separations of a gas from admixture with other gases are important industrial processes. In such processes the objective may be either to obtain a product gas enhanced in a particular gas or from which that particular product gas has an undesired constituent removed therefrom. For example, there are commercial scale processes to separate air into its component gases to obtain nitrogen, oxygen, and argon and for air prepurification processes to pretreat the air prior to use in other processes such as the cryogenic separation of air into its component gases.
More specifically, air separation can be accomplished using adsorption processes, in particular, pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA) type processes. In PSA and VPSA processes, compressed air is pumped through a fixed bed of an adsorbent exhibiting an adsorptive preference for one of the main constituents whereby an effluent product stream enhanced in the non-adsorbed (or lesser adsorbed) constituent is obtained. Compared to more traditional cryogenic air separation processes, adsorption processes for air separation require relatively simple equipment and are relatively easy to maintain. Adsorption processes, however, typically have lower product recovery than many cryogenic processes. For this reason, improvements in the efficiency of adsorption processes remain an important goal. One principal means of improvement is the discovery and development of better adsorbents. Some such adsorbents have led to reduced cycle times within a given adsorption process. According, new equipment capable of meeting the demands of reduced cycle times are required.
There also continues to be a demand for PSA and VPSA plants with lower power consumption. The basic process employs a selective adsorbent to remove at least one component of a gas mixture, employing four basic process steps: (1) adsorption, (2) depressurization, (3) purge and, (4) repressurization. The gas mixture containing the more readily adsorbable component and a less readily adsorbable component is passed through at least one adsorbent bed capable of selectively adsorbing the more readily adsorbable component at a predetermined (upper) adsorption pressure. The gas stream exiting the bed at this upper pressure is now concentrated in the less readily adsorbable component, and is removed for example as product. When the bed becomes saturated with the readily adsorbable component, the bed is thereafter depressurized to a lower desorption pressure for the desorption of the readily adsorbable component, with this gas then discharged from the system. Some processes can include additional steps such as equalization and product pressurization.
Conventional PSA and VPSA processes employ rotary-type positive displacement blowers for either gas pressurization or evacuation in an adsorbent bed. These conventional rotary-lobe blowers typically have lower efficiencies and higher maintenance costs than centrifugal compressors, but they do adapt quite well to the oscillating nature of the pressure swing cycles. FIG. 1 shows an exemplary pressure history of a feed blower pressure demand for a VPSA cycle and FIG. 2 shows an exemplary pressure history of a vacuum blower pressure demand for a VPSA cycle. An attractive feature of rotary-lobe blowers pertaining to an adsorption process is that power consumption is proportional to system pressure requirements. The theoretical power consumption of the blower is directly proportional to the system pressure differential (i.e., has a linear relationship between power consumption and pressure). This linear power response to the system pressure requirements has made rotary-lobe blowers the compression equipment of choice for the PSA and VPSA industry. Rotary-lobe blowers, however, create strong pressure pulsations in the system. Without proper mitigation, pressure pulsations from rotary-lobe blowers are known to cause severe structural damage in downstream process equipment. Although pulsation damper vessels are normally used with rotary-lobe blowers, they do not eliminate the mismatch completely, and a considerable pressure pulsation level is always present in the system.
In the past, centrifugal compressors at fixed speeds with and without inlet guide vanes (IGVs) and with a variable-frequency drive (VFD) have been considered for PSA and VPSA processes because of their higher efficiency when compared to conventional rotary-lobe blowers. FIG. 3 illustrates a typical conventional centrifugal compressor system arrangement. A gearbox 10 with a lube oil system 11 is typically needed to convert the low speed of the induction motor (IM) 12 to the high speed of the centrifugal compressor 1, and the centrifugal compressor needs to run at high speeds for high efficiency. To most effectively use centrifugal compressors in highly dynamic pressure swing cycles, it is necessary to employ IGVs, variable-speed control or a combination of the two. As the PSA or VPSA cycle pressure deviates from the design pressure condition of a fixed-speed centrifugal compressor, the stage efficiency deteriorates substantially, especially when operating at pressure ratios close to 1 (choking). This results in increased average power consumption and a deterioration of the overall average compressor efficiency over the PSA or VPSA cycle. By continuously varying the compressor speeds, however, to match the head requirement of both the pressurizing and evacuating of the adsorbent beds, the compressors can theoretically be operated at their peak efficiencies from 100% design speed to a substantially lower speed. The power consumption now becomes very small, and hence, the average power economy and the overall cycle efficiency is improved dramatically relative to the use of rotary-lobe blowers.
Still, this technology could not be successfully employed in the past. The use of conventional fixed-speed centrifugal compressors with and without IGVs is not ideal because of their limited operating range. A conventional variable-speed centrifugal compressor can have an improved operating range and improved energy savings over the use of IGVs with a reduction in flow, but is unable to rapidly adapt to the transient flow conditions of the PSA or VPSA cycle (due to the large inertias of the gears and large slow running IM rotor).
Centrifugal compressors at fixed speeds with and without IGVs and with a VFD have previously been considered for PSA and VPSA processes. A. Abdelwahab, “Design of A Moderate Speed-High Capacity Centrifugal Compressor with Application to PSA And VPSA Air Separation Processes”, Proceedings of PWR2005 ASME Power, Apr. 5-7, 2005, discusses the fundamentals of a VPSA cycle that makes use of moderate speed direct coupled centrifugal compressors with inlet guide vanes.
Several advances to PSA and VPSA processes have taken place in recent years. Some of these advances include: (a) a significant reduction in the ratio of the top adsorption to bottom desorption pressures, and (b) reductions in the cycle time (typically less than one minute) leading to reduced adsorbent inventories. A significant factor to the total energy requirement of a PSA or VPSA process is this ratio of adsorption to desorption pressures. The delivery pressure during the adsorption period of a bed by the feed air compression device, as well as the suction pressure during the desorption period by an evacuation device, is constantly changing as the cycle progresses. In order to achieve the lowest possible total power consumption for a cycle such as this, it is desirable for the feed compression and evacuation devices to be operated at peak efficiency over a wide range of pressure ratios.
The present invention relates to the application of newly designed high-speed induction motors with variable-speed operation to newly designed pressure/vacuum type adsorption systems with more advanced designs including faster cycle times and reduced power consumption.